1. Field of the Invention
The invention relates to a control device for an internal combustion engine and a control method for an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine mounted on a vehicle, such as an automobile, part of air-fuel mixture or combustion gas present in its combustion chamber may leak into a crank chamber as a so-called blow-by gas. The blow-by gas leaking into the crank chamber may degrade engine oil and, by extension, degrade the internal combustion engine. Then, a blow-by gas recirculation system is provided to return such blow-by gas to an intake passage to be subjected to re-combustion in the combustion chamber during operation of the internal combustion engine.
The blow-by gas recirculation system is formed of a blow-by gas passage, a flow rate control valve, and the like. The blow-by gas passage returns blow-by gas, flowing from the combustion chamber into the crank chamber, back to the intake passage. The flow rate control valve adjusts the flow rate of gas returned through the blow-by gas passage. Then, the flow rate control valve may be, for example, a negative pressure flow rate control valve that uses pressure in the intake passage, that is, intake pressure, to change its opening degree. In the above negative pressure flow rate control valve, the flow rate of gas is controlled on the basis of an intake pressure.
Here, for example, the amount of blow-by gas increases as an engine load increases, and an intake pressure increases as the engine load increases. Thus, the opening degree characteristic of the negative pressure flow rate control valve is regulated so that the flow rate of blow-by gas increases as the intake pressure increases because of an increase in engine load.
On the other hand, in place of the above negative pressure flow rate control valve, as described in Japanese Patent Application Publication No. 2006-207496 (JP-A-2006-207496), there has been known a flow rate control valve, or the like, equipped with an actuator. The flow rate control valve is able to selectively adjust its opening degree. In the above flow rate control valve with an actuator, the opening degree may be adjusted selectively irrespective of an intake pressure, so it is possible to further appropriately adjust the flow rate of gas.
Incidentally, there has been known an internal combustion engine that has a cylinder cut-off mechanism or a variable valve mechanism. The cylinder cut-off mechanism stops the operation of a portion of cylinders among all cylinders. The variable valve mechanism changes various valve characteristics, such as the maximum valve lift, valve opening duration or valve timing of an intake valve.
In an internal combustion engine with no such mechanisms, the relationship between the engine load and the intake pressure is substantially uniquely defined (as the engine load increases, the intake pressure increases (the negative pressure in the intake passage decreases)). Thus, the negative pressure flow rate control valve, of which the flow rate of gas is adjusted on the basis of an intake pressure, is also able to appropriately adjust the flow rate of gas on the basis of an engine load, that is, adjust the flow rate of gas on the basis of the amount of blow-by gas.
However, in the internal combustion engine with any one of the above described mechanisms, the intake pressure varies not only depending on an engine load but also depending on the number of operating cylinders and/or a valve characteristic. For example, in the internal combustion engine with the cylinder cut-off mechanism, the opening degree of a throttle valve is controlled so that, as the number of operating cylinders reduces, the amount of intake air per cylinder increases. Thus, as the number of operating cylinders reduces, the intake pressure in the intake passage increases. Thus, even when the opening degree of the flow rate control valve is the same, the flow rate of gas is reduced as the number of operating cylinders is reduced. In addition, in the internal combustion engine with the variable valve mechanism that changes the maximum valve lift and/or valve opening duration of each intake valve, the amount of intake air may be adjusted by changing the maximum valve lift and/or the valve opening duration in place of adjusting the amount of intake air by the throttle valve. Thus, in this internal combustion engine as well, the intake pressure corresponding to the engine load changes on the basis of a maximum valve lift and/or a valve opening duration. In addition, in the internal combustion engine with the variable valve mechanism that changes the valve timing of each intake valve, the pressure in the intake passage varies by changing the valve timing. Thus, in this internal combustion engine as well, the intake pressure corresponding to the engine load varies on the basis of a valve timing.
Thus, in the internal combustion engine with any one of the above mechanisms, when the above negative pressure flow rate control valve is used, it may be difficult to appropriately control the flow rate of gas. For example, as the flow rate of gas excessively increases, oil component contained in blow-by gas also excessively returns to the intake passage, so there is a concern that the amount of oil consumption inconveniently increases. In addition, as the flow rate of gas excessively reduces, scavenging of the inside of the crank chamber stagnates, so there is a concern that degradation of engine oil is inconveniently accelerated.
On the other hand, the flow rate control valve with an actuator is able to selectively adjust its opening degree irrespective of an intake pressure, so it is possible to appropriately control the flow rate of gas. However, application of the system described in JP-A-2006-207496 to an internal combustion engine with any one of the mechanisms, in which the relationship between the engine load and the intake pressure is not uniquely defined, is not taken into consideration. Therefore, in the internal combustion engine with the cylinder cut-off mechanism or the variable valve mechanism that changes the valve characteristic of each intake valve, when the flow rate control valve with an actuator is provided, there is still room for improvement in controlling the opening degree of the flow rate control valve.